The performance of a cellular network is limited by the amount of inter-cell interference (ICI) experienced by user equipment (UEs). Specifically, UEs at an edge of a cell can experience ICI from adjacent cells at receive power levels similar to that of the receive power from the cell. This is especially true in the case where a frequency reuse factor is one. While a frequency reuse factor of one maximizes a spectral efficiency of the cellular network, the ICI results in a lowering of the signal-to-interference-and-noise (SINR) levels for the UEs at the edges of the cells. Lower SINR for the cell edge UE results in lower transmission rates at any given decoding quality as measured by an error rate.
In orthogonal frequency division multiple access (OFDMA) communication networks, the available spectrum is partitioned into multiple mutually orthogonal groups of subcarriers (frequency bands), often referred to as resource blocks (RBs), i.e., resource units, which are assigned to UEs in each cell in a way that no intra-cell interference occurs as a result of this allocation. Such allocations are usually carried out by a scheduler module that assign a set of RBs to UEs, where those UEs experience high SINR levels so as to maximize the overall network performance and achievable throughput.
The use of multiple-input-multiple-output (MIMO) technology in a cellular network can increase the performance of wireless communication networks. In a MIMO network, a transmitter with MT transmit antennas transmits data to a receiver with MR receive antennas, thereby using a total of MT×MR sub-channels.
In a cellular downlink network, where a base station (BS) also referred to as Node-B or eNodeB in some standards such as 3GPP LTE, transmits data to multiple UEs in the cell, MIMO technology can allow the BS to serve multiple UEs simultaneously while using the single set of time-frequency resource units or RBs.
Employing a hard frequency reuse pattern across multiple cells reduces the ICI experienced by the cell-edge UEs. In a cellular network with a frequency reuse factor larger than one, each cell uses only a fraction of the entire spectrum, and thus, the overall achievable throughput can be lowered despite the potential gain resulting from the reduced amounts of ICI. Fractional frequency reuse is a well known variation of hard frequency reuse patterns.
FIG. 1A shows one possible implementation of the frequency spectrum usage in a fractional frequency reuse network with a reuse factor of three. In this network, an available spectrum 102 is partitioned into four non-overlapping parts. One part 104 is used for transmission between the base station and the UEs near the base station (center UEs). The other three parts 106, 108, and 110 are used for transmissions between the base station and the UEs near the edges of a cell (edge UEs).
FIG. 1B shows one such cellular layout that employs fractional frequency reuse with a reuse factor of three. The fill patterns in FIG. 1A correspond to those in FIG. 1B. All cells use spectrum 104 to transmit to UEs in the center region 112 of their respective cells. When transmitting to UEs at the cell edges, each cell uses the set of RBs specifically assigned to it. In FIG. 1B, cell edge UEs for cells 114 are served using spectrum 108. Similarly, cell edge UEs for cells 116 are served using spectrum 106, and cell edge UEs for cells 118 are served using spectrum 110.
As shown in FIG. 1C, fractional frequency reuse can also be applied in a cellular network where each cell has three sectors. Each sector is served using one 120° directional antenna pointing towards the opposite corner of the cell. UEs in the center regions 112 of each sector are still served using the frequency spectrum 104. The edge regions 122, 124, and 126 of the respective cell sectors, are served using frequency spectrum 106, 108, and 110, respectively.
Inter-cell scheduling is described by Choi et al., “The Capacity gain from Intercell Scheduling in Multi-Antenna Systems,” IEEE Transactions on Wireless Communications, vol. 7, no. 2, February 2008, page 714-725. A cluster is a group of adjacent cells. In an inter-cell scheduling scheme, the cell with a largest achievable rate is given the opportunity to transmit, while the other cells in the same cluster do not transmit.
Examples of base station cooperation schemes, where each UE receives intended signals from multiple transmitting base stations are described by Zhang et al., “Asynchronous Interference Mitigation in Cooperative Base Station Systems,” Transactions on Wireless Communications, vol. 7, no. 1, January 2008, page 155-165.